Semiconducting, insulating, and metallic nanoparticles have attracted considerable interest recently due to their size-dependent, quantum confinement characteristics, which make them attractive for a broad platform of optical, magnetic, and electronic devices. Proposed commercial applications include solid state lighting devices, magnetic recording media, ultra-light video displays, and bio-imaging reagents.
Colloidal nanoparticles (NPs) have diverse, attractive size-dependent electronic, optical and magnetic properties. These colloidal nanoparticles include an inorganic core material surrounded by organic ligand molecules.
Wet processing techniques, for example spin casting, are relatively cheap and easy methods to form dry casts of NPs for device applications. However, these wet methods have serious shortcomings, such as imposing requirements on the deposition surface or limited lateral patterning capacity. The most widely used methods for casting nanoparticle (NP) constituents into densely packed, thermally stable films, such as evaporation-driven self assembly and Langmuir-Blodgett casting, also have recognized serious limitations, including the inability to achieve both large-scale ordering of the nanoparticles as well as robust chemical and structural properties.
Meanwhile, thin films of nanocrystals have been proposed for applications as diverse as solid-state lighting,[1,2] magnetic storage,[3] and catalysis.[4,5] Typically, these thin films of nanocrystals remain permanently attached to the bulk substrates upon which they were initially assembled.
There do exist techniques to construct freestanding nanostructured films, for which the film is assembled over a temporary substrate and the substrate then dissolved.[6] A wide assortment of NP-only assemblies have been reported, but all are either attached to the original substrate or limited to microscopic dimensions.[22] Several groups have produced macroscopic structures of NPs, but only with the aid of chemical crosslinkers or by forming polymer composites.[23,24]
For instance, a composite film of oppositely charged nanoparticles and polyelectrolyte was produced by an electrostatically driven layer-by-layer (LbL) assembly process. However, this LbL method is severely limited because it cannot be used for uncharged nanoparticles. This severely limits the selection of functional materials that may be assembled in this fashion. The production of films comprising one type of nanoparticle via LbL processing requires particles with complementary binding interactions, e.g. electrostatic or covalently coordinated.[7]
What is needed is an approach to fabricating macroscopic structures of nanoparticle-only thin films that achieve both large-scale ordering of the nanoparticles as well as robust chemical and structural properties, but without 1) the aid of chemical crosslinkers or the formation of polymer composites; and preferably simultaneously without 2) imposing requirements on the deposition surface or limited lateral patterning capacity.